1. Field of the Invention
The present invention relates generally to air-cooled, dry-heat exchangers, and more particularly to an assembly for use as a part of an air-cooled, atmospheric water cooling tower for cooling a liquid with currents of air.
2. Discussion of the Prior Art
Processes employing air-cooled, dry external surface heat exchangers operate with elevated fluid-return temperatures as compared with the fluid-return temperatures obtainable by wet evaporative cooling. This results from the fact that system efficiencies are usually less with air-cooled, dry heat exchangers of the same size as an evaporative-type, wet heat exchange. As a consequence, as heat loads increase it is often impractical to use air-cooled dry heat exchangers because of their relative inefficiency.
In certain instances though, the use of wet evaporative heat exchangers presents siting problems because of the necessity or desirability of limiting formation of a visible discharge or plume oftentimes encountered with large conventional evaporative or wet-type water cooling towers. The use of dry towers also offers the advantage of limiting the loss of liquid as a result of the evaporative effect. Furthermore, dry towers allow cooling of fluids other than water, which would preclude the use of an evaporative tower. Fog or plume abatement and the concomitant advantages of dry heat exchange have led to proposals that the dry concept be employed in larger industrial-sized towers, and that evaporative fill and air-cooled heat exchangers be incorporated into a single tower. This is especially attractive because the more efficient wet section of a dual-type tower can be utilized alone when ambient weather conditions permit, but such use can be lessened with the consequent increased initiation or use of the dry section as may be necessary for plume abatement under particular cold or cooler climate conditions.
Exemplary in this respect is the fact that a wet surface water cooling tower having a selectively usable dry surface heat exchanger unit as an integral part thereof may be locatable in closer proximity to a roadway than would otherwise be the case because of the plume abatement afforded by the added dry section. Without adequate plume abatement, location of a wet cooling tower in proximity to a roadway could result in obstructed visibility hazards which would preclude such positioning of the tower.
Another problem associated with finned tube heat exchangers, and especially those of relatively large capacity, stems from the high cost of producing the heat exchangers and the water delivery and return mechanisms especially adapted for use therewith. As such, the more efficient and less expensive evaporative water cooling towers are preferred over air-cooled heat exchangers even in situations where the advantages of dry heat exchange would otherwise be manifest.
An attempted solution to the foregoing problems is set out in U.S. Pat. No. 3,995,689, to Cates and assigned of record to the assignee of the present application. In the '689 patent, an air-cooled atmospheric heat exchanger is illustrated as including a plurality of U-shaped members arranged in a pack, wherein each member is preformed out of a low cost synthetic resin material such as polyvinyl chloride. Each U-shaped member presents a pair of side-by-side panels that are connected together by a common, longitudinally extending upper edge, and the panels of each member present front and rear, vertically extending side edges that are sealed to the side edges of the panels on adjacent members in the pack. The space between the panels of adjacent members is thus sealed along the front and rear edges to define respective vertical water passages that are separate from the air passages defined between the panels of each individual member, and each panel presents a wettransfer surface adapted to transfer heat between the water and the panel, and a dry-transfer surface adapted to transfer heat between the panel and air passing through the pack.
The solution proposed by the '689 patent did not find commercial acceptance for several reasons. For example, negative pressures tend to develop within the water passages of the pack and cause the relatively thin panels of the members to collapse toward adjacent members, disfiguring the panels and causing channeling of the water within the pack. As such, the efficiency of a heat exchanger constructed in accordance with the '689 patent is substantially compromised.
In an attempt to overcome the foregoing adverse effect created by the siphoning of water through the heat exchanger, the '689 patent teaches that air can be intentionally entrained in the hot water entering the top of the pack to slow the travel of the water through the heat exchanger and preclude the buildup of negative pressures between the panels of adjacent members. The negative pressures that built up between the sheet sections defining the upright water passages were so great that the sheet sections collapsed toward one another thus creating restrictions in downward flow of water and limiting the overall efficiency of the equipment. The patentee in the '689 patent sought to eliminate leaking of water from the water passages defined by the pack sheets by causing air to be entrained in the water entering the water passages. However, this was found to be only partially satisfactory, and the problem was exacerbated by difficulties in effectively sealing the interengaging edges of the sheets, particularly at the air inlet face of the patent.
Another drawback experienced in the construction of the proposed heat exchanger of the '689 patent resides in the requirement that both the front and rear edges of each water flow passage be sealed to prevent air and water from leaking into or out of the water passages defined between the members. Although it was theorized that the siphoning effect of water passing through the heat exchanger would combine with the positive pressure of air flow through the pack to hold the rear edges of adjacent members against one another, it was found that the front edge of each water flow passage in that air impinging on the front edge tended to enter the water passage and become drawn into the water passing between adjacent members. When air was thereby allowed into the water passage at any point along the length thereof, the air affected the movement of water between the members, creating channeling of the liquid flow.
Other problems also existed in constructing a heat exchanger in accordance with the '689 patent. For example, in order to support the members of each pack, the members were provided with aligned openings within which a support member was received, and the members had to be sealed at the openings to prevent air and water from leaking into or out of the water passages defined between the members. Further, the construction suggested in the '689 patent employed spacers protruding into the air passages such that air pressure between the front and back of the pack dropped substantially, thus adversely affecting the efficiency of the heat exchanger.